Staurosira construens

Valves are cruciform, 5-9 µm wide and 6-15 µm long. Valves with rostrate to subcapitate ends. The central margin is inflated and narrowly or widely curved. Valve face flat, or slightly undulate due to raised costae. The valve face/mantle junction forms a sharp angle. Abvalvar edge of mantle parallel to the valve face/mantle junction. In girdle view, frustules are rectangular, but show the prominent inflation and form ribbon-like colonies, joined by linking spines. The colonies are attached to the substratum at one end by a frustule attached by a mucilage pad, or colonies may be planktonic. The axial area is linear to lanceolate, much wider at the valve central area in some specimens. Striae are distinct, alternate, and composed of lineolae decreasing in size from the valve face/mantle edge to both the central sternum and the valve mantle; 14-16 in 10 µm. The lineolae bear finely branched volae. Striae are curved and vary from parallel to radiate in the central area to slightly radiate to parallel toward the valve ends. Sometimes shorter striae intercalated with longer striae are present toward the valve face edge in the central inflations. Striae extend onto the valve mantle. The costae are broad. Spines are spatulate, hollow and some possess terminal digitations that connect to the neighboring valve lineolae. Spines are present along the valve face edge, except at the apices, and always located on the costae, between striae. Well-developed, ocellulimbus type, apical pore fields with round poroids are present. Poroids are each frequently surrounded by a whitish rim, presumably siliceous in nature. Apical pore fields are located on the transition between valve face/mantle. Rimoportula are absent. Scab-like structures, or blisters, are absent. Copulae, or girdle bands are open and do not bear perforations. Valvocopula only slightly wider than copulae.

Populations from North America often contain morphological variants that occur together in a single locality. It is uncertain whether these variants are the product of sexual reproduction occurring in a single population, ecotypes resulting from adaptation to local environmental conditions, or whether they in fact represent different species. The type material of Ehrenberg also contains such variation (P. Hamilton, pers. comm.).